This invention relates to the edge fastening of panels or sheets of material, and more particularly to such fastening including both adhesive joints and stir-friction welded joints.
The fastening of composite materials to metallic materials or to other composite materials is complicated by the fact that composite materials including thermoset materials or reinforcements cannot, in general, be welded, because the matrix material, being organic, chars rather than melting, and the reinforcing material cannot be raised to a high enough temperature to fuse, without destroying the underlying matrix. Composite materials are very advantageous for many purposes, as they combine high strength with low weight. Practical use of composite materials often requires that they be fastened to other composite structures or to metallic materials.
In general, composite structures can be fastened to other structures by either the use of fasteners or by co-curing. The use of fasteners such as bolts or screws creates differential stresses along the joint, and creates a sealing problem in those cases in which fluids, especially cryogenic fluids, are to be contained. These problems are exacerbated when there is a difference between the temperature coefficients of expansion of the materials being fastened and or of the fastener. One solution for the fastening of composite materials using fasteners is described in U.S. Pat. No. 6,171,009 B1 issued Jan. 9, 2001 in the name of Wright, and another solution is described in U.S. patent application Ser. No. 09/690,964, filed Oct. 18, 2000 in the name of Gudaitis et al.
The use of fasteners can be effective, but the parts count may be high, especially when seals are associated with the fasteners.
Co-curing is a useful method for fastening composites to metallic structures without the need for fasteners. In co-curing, the metallic piece and the uncured composite with its reinforcement (if any) are juxtaposed, and the uncured resin or matrix material is carried onto the surface of the metallic material. The uncured resin is cured, often at high temperatures under vacuum, and becomes integral with both the reinforcement and the metallic material. U.S. Pat. No. 5,441,219, issued Aug. 15, 1995 and U.S. Pat. No. 5,427,334, issued Jun. 27, 1995, both in the name of Rauscher, Jr., describe the fabrication of metal-composite tank structures using co-curing. The combination of high temperature and vacuum conditions for curing often requires the use of an autoclave and special fixtures. Autoclaves cannot be used under all conditions, especially when large assemblages are to be cured. Such large assemblages may be, for example, large fluid storage tanks, the bodies or wings of aircraft, hulls of ships, and the like.
Improved fabrication methods are desired for composite-to-composite and composite-to-metallic joints.
A method for joining a composite sheet to a metallic sheet includes the step of procuring a composite sheet having a particular thickness, and which defines an edge which is to be fastened to the metallic sheet. An elongated metallic structure is procured. The elongated metallic structure has a cross-section including a common portion or tang and a bifurcation defining a pair of parallel tines. The distal ends of the tines (as seen in cross-section) define an elongated slot in the elongated metallic structure. The elongated slot has a width not less than the thickness of the composite sheet. According to an aspect of the method, the edge of the composite sheet is adhesively fastened into the slot, to make a structural joint between the composite sheet and the elongated metallic structure. The adhesive fastening may be by any kind of adhesive, and in particular the adhesive may be a heat-cured or heat-bonded resin. At this point in the fabrication, the composite sheet is fitted with an elongated metallic edge. The metallic edge of the composite sheet (the common portion of the elongated metallic structure) is stir-friction welded or fused to the metallic sheet. The stir friction welding or fusing occurs at lower temperatures than conventional fusion welding, so the adhesive joint is not likely to be damaged by the welding temperature.
In a particular mode of the method according to the invention, the adhesive fastening step includes the step of placing the edge of the composite sheet into the slot in the elongated metallic structure, followed by the step of injecting adhesive into the region between the slot and the composite sheet. This injection step, of course, may be followed by a heat-curing of the adhesive step, if appropriate.
In another particular mode of the method according to the invention, the step of procuring an elongated metallic structure includes the step of procuring an elongated metallic structure having a cross-section including a common, tang or heel portion and a bifurcation defining a pair of parallel tines and at least one through aperture extending through a tine at a location remote from the distal ends of the tines. The edge of the composite sheet is placed in the slot, and adhesive is then injected through the aperture into at least a portion of the region between the slot and the composite sheet. In another mode of the invention, the step of procuring a sheet of composite material includes the procuration of a planar sheet of composite material.